As a conventional inkjet recording system, a continuous system (for example, see JP-B-41-16973 (“JP-B” means examined Japanese patent publication)) that always pressure-sprays ink as a droplet from a nozzle by ultrasonic vibration, charges a flying ink droplet, and polarizes the ink droplet by an electric field, to continuously record an image. As a drop-on-demand system or the like for timely flying an ink droplet, an electrohydrodynamic system (for example, see JP-B-36-13768 and JP-A-2001-88306 (“JP-A” means unexamined published Japanese patent application)), which applies a potential across an ink ejecting portion and a sheet of recording paper, and attracts an ink droplet from the ink ejecting port by electrostatic force, to cause the ink droplet to adhere to the sheet of recording paper; a piezo-conversion system, or a thermal conversion system (for example, see JP-B-61-59911) such as a bubble jet (registered trademark) system (thermal system), are known.
As a drawing system for a conventional inkjet apparatus, a raster scan system, for displaying one image by using scan lines, has been used.
However, the conventional inkjet recording system poses the following problems.
(1) Difficulties in Ejection of an Ultrafine Droplet
Currently, in an inkjet system (piezo system or thermal system) that is practically and popularly used, a minute amount of liquid, smaller than 1 pl, cannot be easily ejected. This is because the pressure required for ejection increases as the diameter of the nozzle decreases to be finer.
In an electrohydrodynamic system, for example, a nozzle inner diameter described in JP-B-36-13768 is 0.127 mm, and the opening diameter of a nozzle described in JP-A-2001-88306 is 50 to 2000 μm, preferably 100 to 1000 μm. Therefore, it has been considered that an ultrafine droplet of size 50 μm or less cannot be ejected.
As will be described below, in an electrohydrodynamic system, extreme accuracy is required to control a driving voltage to realize a fine droplet.
(2) Luck of Landing Accuracy (Touchdown Accuracy)
Kinetic energy given to a droplet ejected from a nozzle decreases in proportion to the cube of the droplet radius. For this reason, a fine droplet cannot possess kinetic energy that is sufficient to withstand air resistance, and accurate landing cannot be expected, because of air convection or the like. In addition, as the droplet becomes fine, the effect of surface tension increases, which makes the vapor pressure of the droplet become high, and drastically increases the amount of evaporation. With this being the case, the mass of the flying fine droplet is considerably lost and even the shape of the droplet can hardly be kept in landing.
As described above, miniaturization and precision of a droplet and increased accuracy of landing positions thereof are incompatible subjects so that both cannot be easily realized at once.
Poor accuracy of landing positions not only deteriorates printing quality but also poses a considerable problem especially when the circuit pattern is drawn by using conductive ink, such as with an inkjet technique. More specifically, poor position accuracy not only makes it impossible to draw a wire having a desired width but also may cause disconnection or short-circuiting.
(3) Difficulties in Decrease of the Driving Voltage
When an inkjet technique according to an electrohydrodynamic system (for example, JP-B-36-13768), which is an ejection system different from the piezo system or the thermal system, is used, kinetic energy can be given by an applied electric field. However, since the apparatus is driven by a high voltage of over 1000 V, decreasing the size of the apparatus is limited. Although an apparatus described in JP-A-20001-88306 describes that a voltage of 1 to 7 kV is preferably used, a voltage of 5 kV is applied to in an example therein. To eject an ultrafine droplet and realize high throughput, introduction of multi-heads and high-density arrangement of heads are important factors. However, since the driving voltage in a conventional electrohydrodynamic inkjet system is very high, i.e., 1000 V or more, decreasing size and increasing density are difficult, because of leakage of current between the nozzles and interference between the nozzles, and decrease of driving voltage is a problem to be solved. In addition, a power semiconductor using a high voltage of more than 1000 V is generally expensive and has poor frequency responsiveness. In this case, the driving voltage is the total voltage applied to nozzle electrodes, and the sum of the bias voltage and the signal voltage (in this specification, the driving voltage means the total applied voltage, unless otherwise noted). In a conventional technique, a bias voltage is increased to decrease a signal voltage. However, in this case, a solute in an ink solution tends accumulate on nozzle surfaces by the bias voltage. The ink is fixed due to, for example, electrochemical reaction between the ink and the electrodes, and clogging of the nozzles or wasting of the electrodes disadvantageously occurs.
(4) Restriction of Usable Substrate and Layout of the Electrode
In a conventional electrohydrodynamic inkjet system (for example, JP-B-36-13768), a sheet of paper is assumed to be a recording medium, and a conductive electrode is required on the rear surface of the printing medium. There is a report that printing can be performed by using a conductive substrate as the printing medium, which, however, poses the following problem. When a circuit pattern is formed by an inkjet apparatus using conductive ink, if printing can only be performed on a conductive substrate, the circuit pattern cannot be directly used as an interconnection, and the application is considerably limited. For this reason, a technique that can also perform printing on an insulating substrate, such as glass and plastic, is needed. In addition, some conventional techniques in which an insulating substrate, such as glass, are used, is reported. However, an electrically conductive film is formed on the insulating substrate, or a counter electrode is arranged on the rear surface of the insulating substrate with decreasing the thickness of insulating substrate, so that a usable substrate or the layout of electrodes is limited.
(5) Instability of Ejection Control
In a conventional drop-on-demand electrohydrodynamic inkjet system (for example, JP-B-36-13768), a system that performs ejection control by turning on/off an applied voltage, or an amplitude modulation system that performs ejection control by applying a DC bias voltage to some extent and superposing a signal voltage thereon, is used. However, since the total applied voltage is high, i.e., 1000 V or more, the power semiconductor device to be used must be one that is expensive and poor in frequency responsiveness. Further, a method of applying a predetermined bias voltage, which is not enough to start ejection, and superposing a signal voltage on the bias voltage, to perform ejection control, is frequently used. However, when the bias voltage is high, aggregation of particles in ink is advanced in use of pigmented ink when ejection pauses; a nozzle is apt to be clogged by electrochemical reaction between electrodes and the ink, or other phenomena apt to occur. Thus, there are problems that time responsiveness when the ejection is restarted is poor, and the amount of liquid is disadvantageously unstable after the ejection pauses.
(6) Complexity of Structure
A structure achieved by a conventional inkjet technique is complex and is manufactured at high cost. In particular, an industrial inkjet system is very expensive.
Important design factors for a conventional electrohydrodynamic inkjet, in particular an on-demand electrohydrodynamic inkjet, are conductivity of the ink solution (e.g., resistivity of 106 to 1011 Ωcm), surface tension (e.g., 30 to 40 dyn/cm), viscosity (e.g., 11 to 15 cp), and as an applied voltage (electric field), voltage applied to the nozzles and distance between the nozzles and the counter electrodes. For example, in the above conventional technique (JP-A-2001-88306), to form a stable meniscus to perform preferable printing, the distance between a substrate and nozzles is preferably set at 0.1 mm to 10 mm, more preferably 0.2 mm to 2 mm. A distance less than 0.1 mm is not preferable, as a stable meniscus cannot be formed.
Relationship between the nozzle diameter and the droplet to be generated is not made clear. This is mainly because a droplet attracted by an electrohydrodynamic system is attracted from the semilunar top (called a Taylor cone) of liquid formed by electrostatic force and forms a fluid jet having a diameter smaller than the nozzle diameter. For this reason, a nozzle diameter that is large, to some extent, has been allowed, to reduce clogging in the nozzle (for example, JP-A-10-315478, JP-A-10-34967, JP-A-2000-127410, JP-A-2001-88306, and the like).
A conventional electrohydrodynamic inkjet system uses electrohydrodynamic instability. FIG. 1(a) shows this manner as a schematic diagram. At this time, as an electric field, an electric field E0, generated when a voltage V is applied across a counter electrode 102, which is arranged at a distance h from a nozzle 101, is set. When a conductive liquid 100a stands still in a uniform electric field, electrostatic force acting on the surface of the conductive liquid makes the surface instable, thereby promoting growth of a Taylor cone 100b (Taylor cone phenomenon). A growth wavelength λc set at this time can be physically derived, and is expressed by the following equation (e.g. GAZOU DENSHI JYOHOU GAKKAI, Vol. 17, No. 4, 1988, pp. 185-193):
                              λ          C                =                                            2              ⁢              πγ                                      ɛ              0                                ⁢                      E            0                          -              2                                                          (        1        )            wherein γ is surface tension (N/m), ∈0 is vacuum dielectric constant (F/m), and E0 is intensity of the electric field (V/m). Reference symbol d denotes a nozzle diameter (m). The growth wavelength λc means the shortest wavelength of a wave that can grow in waves generated by electrostatic force acting on the surface of the liquid.
As shown in FIG. 1(b), when the nozzle diameter d (m) is smaller than λc/2 (m), growth does not occur. More specifically,
                              d          >                                    λ              C                        2                          =                  πγ                                    ɛ              0                        ⁢                          E              0              2                                                          (        2        )            is a condition for ejection.
In this case, E0 denotes the electric field intensity (V/m) obtained assuming that parallel flat plates are used. Then, following equation is obtained, representing the distance between the nozzle and the counter electrode by h (m), and the voltage applied to the nozzle by V.
                              E          0                =                  V          h                                    (        3        )            Therefore,
                    d        >                              πγ            ⁢                                                  ⁢                          h              2                                                          ɛ              0                        ⁢                          V              2                                                          (        4        )            is derived.
When the surface tension is given by γ=20 mN/m and γ=72 mN/m, the electric field intensity E required for ejectioin based on the idea of a conventional method is plotted with respect to the nozzle diameter d. The result is shown in FIG. 2. According to the idea of the conventional method, the electric field intensity is determined by the voltage applied to the nozzle, and by the distance between the nozzle and the counter electrode. For this reason, a reduction in nozzle diameter requires an increase in the electric field intensity required for ejection. In a conventional electrohydrodynamic inkjet, when the growth wavelength λc is calculated under typical operation conditions, i.e. a surface tension γ of 20 mN/m and an electric field intensity E of 107 V/m, a value of 140 μm is obtained. Accordingly, as the limit nozzle diameter, a value of 70 μm is obtained. That is, under the above conditions, even if an electric field intensity of 107 V/m is used, when the nozzle diameter is 70 μm or less, ink is not grown unless a process of applying back pressure to forcibly form a meniscus is performed, and it is considered that an electrohydrodynamic inkjet is not established. More specifically, a fine nozzle and a decrease in driving voltage are considered to be incompatible subjects. For this reason, as a conventional measure for a decrease in voltage, a method to achieve a decrease in voltage by arranging the counter electrode just in front of the nozzle, to shorten the distance of the nozzle and the counter electrode is employed.